1. Field of the Invention
The present invention relates to a dot printing method and apparatus, wherein printing means having a plurality of columns each consisting of a plurality of dot printing elements is used and the printing means and recording medium are moved relative to each other to perform dot matrix printing (including image recording).
2. Description of the Prior Art
Dot printing is widely performed in a printer of thermal type using thermal paper or in a printer of thermal transfer type using normal paper, or, again in an ink-jet printer.
Dot printing is classified into serial printing for printing in units of columns, and line printing for simultaneously printing a plurality of columns or a line.
A conventional serial printing method will be described with reference to FIGS. 1 to 3.
Referring to FIG. 1, a printing head 1 has a column 2 consisting of a vertical array of a plurality of (7) dot printing elements C1 to C7. Wires C1 to C7 for individually driving the elements C1 to C7 are connected thereto. A common ground line G is connected to the elements C1 to C7.
The printing head 1 is moved along paper in a direction of arrow while the dot printing elements C1 to C7 are turned on or off at proper timings. Thus, a character (which may include a number, symbol, or figure) such as a number "7", for example, is printed as illustrated in FIG. 2.
FIG. 3 is a timing chart showing the drive timings of the dot printing elements C1 to C7 when the number "7" in FIG. 2 is printed. The number "7" is printed by driving the element C1 at all steps t1 to t5 and the elements C2 to C7 at steps as illustrated in FIG. 3. When a time period of each step is represented by t, the element drive time in each step, i.e., the heating time in a thermal printer or the ink injection time in an ink-jet printer is set to be t/2 which is half the period of each step. Thus, printing at desired density is performed.
When a high-speed printer, for example, a printer having a double speed is to be realized with such a single column dot printer, a method can be adopted wherein the element drive time is reduced to t/4 which is half that of the normal printer and the moving speed is doubled so that the time period of each step is kept at t/2. However, with this method, the drive time of each dot, i.e., the heating time or the ink injection time is reduced to half. This results in light and unclear printing, as shown in FIG. 5.
In order to keep the printing density at a normal level, another method can be adopted wherein the element drive time is kept at t/2 which is the same as that in the normal printer but the overall printing process is performed at high speed (e.g., double speed). However, with this method, the head moving distance per step (time t) is increased (e.g., doubled). Then, the width of a printing column (width of columns L1, L2, ...) is doubled, and characters are elongated laterally to have a double width.